The present invention relates to a catalyst for dehalogenation of a halide and a method for dehalogenating a halide using this catalyst. More particularly, the present invention relates to a catalyst for dehalogenation of a halide such as a chloride, iodide, or bromide of silicon, titanium, zirconium, hafnium, niobium, tantalum, molybdenum, tungsten, tellurium, cadmium, etc., as well as a method for dehalogenation of such halides using this catalyst.
In the electronics industry, there is a great demand for ultrapure forms of the elements mentioned above which are generally referred to as "new metals": high-purity silicon is used in large quantities as substrates for LSI; high-purity niobium is used as a superconducting material; and high-purity molybdenum, tungsten, etc., are used as metal or metal silicide targets in the manufacture of LSIs. The use of these "new metals" is anticipated to expand further in the years to come.
Commercial production of these new metals in high purity starts with a crude material of low purity which is halogenated and distilled to attain a product of higher purity, which is then subjected to thermal decomposition or hydro-decomposition to produce a product having the desired ultrahigh purity. A halide of higher halogenation degree is not easy to decompose and causes such disadvantages as low yield and low rate of reaction and the need to employ high reaction temperatures. It is therefore advantageous to use a halide of lower halogenation degree as the starting material In addition, if the halide of lower halogenation degree generates a halide of higher halogenation degree as a by-product of decomposition reaction, it is advantageous to re-convert this by-product into a halide of lower halogenation degree for further use as the starting material.
It is advantageous for the purposes of increasing the rate and yield of reaction and of allowing the reaction to proceed under mild conditions that trichlorosilane rather than tetrachlorosilane in the production of high-purity silicon, and niobium tetrachloride rather than niobium pentachloride is hydrodecomposed in the production of high-purity niobium. Furthermore, the halides to be handled are corrosive and it is of extreme importance to reduce their corrosive nature by employing lower reaction temperatures.
Therefore, halides of higher halogenation degree are converted to those of lower halogenation degree before they are subjected to subsequent steps and the reactions employed for this purpose are collectively referred to as "dehalogenating reactions".
One prior technique of dehalogenation of halides involves dehalogenating tetrachlorosilane into trichlorosilane by hydrogenation conducted in the presence of hydrogen using copper or copper chloride as a catalyst (see, for example, Japanese patent application (OPI) Nos. 16915/83, 73617/81 and 45919/84}(the term "OPI" used herein means a published unexamined Japanese patent application). A method is also known that performs the same reaction using a catalyst containing a metal of the platinum group (Japanese Pat. Publication No. 10532/80).
However, in the former method, copper chloride forms during reaction if copper is used as a catalyst (to say nothing of the case where copper chloride is used as a catalyst), and the copper chloride, which is volatile under the reaction conditions employed, enters trichlorosilane to lower the purity of the product and cause excessive consumption of the catalyst. Hence, the copper or copper chloride catalyst is not suitable for prolonged service.
According to Japanese Pat. Publication No. 10532/80, a platinum group metal used as a catalyst component is supported on activated carbon, alumina, or silica, and it can safely be assumed that the carriers actually used in this reference method are porous activated carbon, .gamma.-alumina, and common porous silica gel, respectively. In the catalyst under consideration, the platinum group metal is not consumed as fast as copper, but on the other hand, the carrier is consumed to cause a rapid decrease in the catalytic activity. Therefore, this catalyst is also unsuitable for prolonged service.
It is very important for successful industrial manufacturing operations that products of consistent quality are continuously produced for a prolonged period of time. In view of this aspect, the catalysts of the prior arts are not suitable for industrial use.